Exposure apparatus with a pulsed laser

ABSTRACT

An exposure apparatus comprising (a) irradiating means for illuminating a mask with laser light from an excimer laser and (b) a projection optical system for projecting a pattern of the mask onto a substrate with the laser light, wherein a characteristic of the projection optical system is measured by use of a harmonic of a predetermined laser, and wherein the laser light from the excimer laser has a wavelength corresponding to that of the harmonic of the predetermined laser.

This application is a divisional of application Ser. No. 09/285,672,filed Apr. 5, 1999 now U.S. Pat. No. 6,348,357, which is a divisional ofapplication Ser. No. 08/577,474, filed Dec. 22, 1995, issued as U.S.Pat. No. 5,969,799, on Oct. 19, 1999.

FIELD OF THE INVENTION AND RELATED ART

This invention relates generally to an exposure apparatus and a devicemanufacturing method using the same. More particularly, the invention isconcerned with an exposure apparatus as a stepper, for example, and adevice manufacturing method using the same, suitable applicable to themanufacture of various devices such as ICs, LSIs, CCDs, liquid panels ormagnetic heads, by, for example, illuminating a circuit pattern of areticle with pulse light from an excimer laser, for example, and byprojecting the illuminated circuit pattern onto a wafer.

Recently, in an exposure method us an ultra-high pressure Hg lamp,various attempts have been made to enhance the resolution by changingthe exposure wavelength to i-line from g-line. Also, many proposals havebeen made to use pulse light of a shorter wavelength, as represented byan excimer laser, to increase the resolution. The use of shortwavelength light is effective to enlarge the depth of focus with thereduction in wavelength, to thereby improve the resolution.

The emission spectrum of an excimer laser, in a case of a KrF excimerlaser, for example, is about 300 pm (full width at half maximum), andthis is sufficiently small as compared with the full width wavelength oflight from conventional ultra-high pressure Hg lamps. Taking the qualityinto account, the optical material usable in a reduction projection lensof an exposure apparatus which uses light in the deep ultraviolet regionmight be synthetic quartz or fluorite only. For this reason, takingchromatic aberration into account, even with a spectral band width of300 pm, which is sufficiently small as compared with that of ultra-highpressure Hg lamps, the band width has to be reduced (band-narrowed)further by about two digits. Namely, the full width at half maximum hasto be not greater than 3 pm (0.003 nm).

For a reduction of the spectral band width, generally, a band narrowingunit having a dispersion clement such as an etalon or grating is used toband narrow a laser light to a spectral band width of about 1 pm. Whilethe emission center wavelength of the thus band narrowed laser light maydiffer with the laser medium used, usually a wavelength which is in thevicinity of the emission gain by which a maximum output is obtainable isselected. For example, in a case of a KrF excimer laser, it is close to248.35 nm, which is the center of the emission gain.

On the other hand, recent semiconductor device manufacturing apparatusesare required to provide a high resolution and yet a high throughput.Generally, the resolution depends on the numerical aperture (N.A.) of aprojection lens system. Further, for a higher throughput, the size of achip has become larger. This necessitates a projection lens systemhaving a large N.A. and yet a wide field angle.

However, enlargement of the N.A. or the field angle of a projection lenssystem directly leads to a difficulty in lens design. Also, it resultsin a narrowed tolerance to production, making the lens manufacture moredifficult. For the manufacture of a projection lens system, generally, anumber of lenses are combined into a projection lens, and trial printingtests are carried out by using that projection lens system. Aberrationsof the projection lens system are detected on the basis of the resultsof trial printing, and correction of the lens system is performed.However, this process needs the skill of an operator and takes muchtime. Thus, the throughput is low. Also, it is practically difficult tocorrect all products (projection lens systems) precisely and exactly.

As for a method of quantitatively measuring the performance of aprojection lens system having a combination of a number of lenses, thereis a method in which an interferometer is used to detect aberrationsand, on the basis of the detection, the lens system is corrected.Interferometers use a laser as a light source. However, in a case wherean excimer laser is used in an exposure apparatus as a light source,there would be no laser source which is suitably usable in aninterferometer and which has the same wavelength as that of the excimerlaser. The excimer laser of the type used in the exposure apparatus maybe used as a light source for the interferometer. However, the coherencyof excimer lasers is not high, and they are not suited to be used as alight source of an interferometer. Additionally, because excimer outputsis difficult to attain. Thus, with an interferometer using an excimerlaser, it is difficult to obtain good precision in inspection of theperformance of a high-quality projection lens system having a large N.A.and/or a large field angle.

Gas lasers such as a He—Ne laser have good coherency and they providecontinuous wave emission. Thus, gas lasers may suitably be used as alight source of an interferometer. However, their wavelength differsconsiderably from that of excimer lasers. Thus, while taking chromaticaberration or film characteristics into account, it is difficult to usegas lasers for inspection of a projection lens system designed for usewith the wavelength of an excimer laser.

In the vicinity of 248.35 nm, which is the center wavelength of anordinary KrF excimer laser, there is a double harmonic wave (a wavehaving a wavelength a half of its original wavelength) of an argon ionlaser, which can be produced by using a secondary harmonic waveproducing element. However such harmonics have a wavelength of about 100pm, which is quite different from that of the KrF excimer laser. Thus,it cannot be used as a light source for an interferometer for inspectionof a projection lens system designed for use with a band narrowedexcimer laser.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide animproved exposure apparatus.

It is another object of the present invention to provide an improveddevice manufacturing method.

In accordance with an aspect of the present invention, there is providedan exposure apparatus, comprising: irradiating means for projectingpulse light to a mask; and a projection optical system for projecting apattern of the mask onto a substrate, wherein the pulse light has anadjusted wavelength such that it substantially coincides with awavelength of laser light of a continuous wave.

In accordance with another aspect of the present invention, there isprovided an exposure apparatus, comprising: an irradiation opticalsystem for projecting pulse light of a wavelength 248.25 nm to a mask;and a projection optical system for projecting a pattern of the maskonto a substrate.

In accordance with a further aspect of the present invention, there isprovided a device manufacturing method, comprising the steps of:providing a light source adapted to supply pulse light of a wavelength248.25 nm; and projecting the pulse light to a mask so that a pattern ofthe mask is projected through a projection optical system to asubstrate.

In accordance with a yet further aspect of the present invention, thereis provided a device manufacturing method, comprising the steps of:providing a light source adapted to supply pulse light; and projectingthe pulse light to a mask so that a pattern of the mask is projectedthrough a projection optical system to a substrate, wherein the pulselight has an adjusted wavelength such that it substantially coincideswith a wavelength of laser light of a continuous wave.

The projection optical system to be used in the present invention mayinclude a lens assembly, a mirror assembly or a combination of a concavemirror and a lens assembly.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and diagrammatic view of a main portion of anexposure apparatus according to an embodiment of the present invention.

FIG. 2 is a graph for explaining the relation between the output and theemission wavelength of a laser light source, in an embodiment of thepresent invention.

FIG. 3 is a schematic and diagrammatic view of a main portion of anembodiment of the present invention.

FIG. 4 is a schematic and diagrammatic view of a main portion of anembodiment of the present invention.

FIG. 5 is a flow chart of device manufacturing processes, in anembodiment of the present invention.

FIG. 6 is a flow chart of a wafer process. in the sequence of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic and diagrammatic view of a main portion of anexposure apparatus according to a first embodiment of the presentinvention. Denoted in the drawing at 1 is light source means whichcomprises, for example, a KrF excitner laser for providing pulsed laserlight. This excimer laser 1 provides pulse light of a wavelength ofabout 248 nm in the deep ultraviolet region. Denoted at 12 is a bandnarrowing unit which includes a band narrowing element, such as anetalon or grating, for example, for band narrowing the laser light fromthe excimer laser 1. Denoted at 3 is a wave meter for detecting thewavelength of pulse light from the excimer laser 1, and denoted at 4 isa beam shaping optical system which serves to expand the beam diameterof the laser light. Denoted at 5 is an illumination optical system foruniformly illuminating a device pattern on the surface (surface to beilluminated) of a reticle R, placed on a reticle stage 10, with thelaser light from the beam shaping optical system 4. Through thisillumination optical system 5, the reticle surface is illuminated with auniform illuminance distribution.

Denoted at 6 is a projection optical system (projection lens system) forprojecting, in a reduced scale, the circuit pattern (device pattern) onthe reticle R surface onto the surface of a waver W placed on a waferstage 7. Denoted at 9 is a controller which is operable to set andcontrol various process conditions necessary for the projection exposureof the wafer W surface. Denoted at 11 is a laser controller forcontrolling the light source means 1 and the band narrowing unit 2 inaccordance with the process conditions, set by the controller 9, so thatthe light source 1 provides pulse laser light of a predeterminedwavelength.

In this embodiment, for setting process conditions necessary forprojection of the circuit pattern, on the reticle R surface, onto thewafer W surface, for exposure of the latter, initially an operatorinputs, into the controller 9, the necessary resolution as required forthe projection exposure as well as the necessary pulse energy and pulsenumber as calculated from, for example, the sensitivity of a resistapplied to the wafer W. In response, in accordance with the appliedprocess conditions, the controller 9 operates to determine optimum ordesirable conditions for the emission of laser light from the lightsource means 1.

The emission conditions may include the energy of laser light per pulse,the emission frequency and the emission center wavelength, for example.The center wavelength of the laser light to be emitted may be at a levelas automatically set by the laser controller 11, or it may be at a levelonce set by the controller 9 and then transmitted to the lasercontroller 11. The laser controller 11 continuously monitors the emittedlaser light by use of the wave meter 3 so that the emission wavelengthfrom the excimer laser 1 is maintained at the set emission wavelength.Also, the laser controller 11 controls the band narrowing clement (notshown) within the band narrowing unit 2, so that the wavelength of thepulse laser light is controlled at the set wavelength.

As regards the center wavelength to be set by the controller 9, in thisembodiment, in a case where the light source means 1 used in theexposure apparatus comprises a KrF excimer laser, it is set at awavelength 248.25 nm, which corresponds to the wavelength of the doubleor secondary harmonics of an argon ion laser (wavelength 496.5 nm) of acontinuous emission type. While an argon ion laser has several emissionlines, a wavelength of double harmonics of 496.5 nm having a relativelyhigh power is close to the emission wavelength of the KrF excimer laser.Thus, the wavelength of exposure light in the exposure apparatus of thisembodiment is adjusted or registered with that wavelength.

The wavelength by which a maximum output is obtainable from the excimerlaser 1, having been band narrowed by the band narrowing unit 2, isabout 248.35 nm which is at the center of the gain. Thus, setting theemission wavelength of the excimer laser 1 at 248.25 nm results in adeviation of the wavelength by about 0.1 nm from the wavelength by whicha maximum output is obtainable. However, according to experiments madeby the inventors of the subject application, a relation such asillustrated in FIG. 2 has been obtained between the laser emissionwavelength and the output.

More specifically, it has been found that, in a band narrowed KrFexcimer laser, even if the set emission wavelength deviates by an orderof 0.1 nm from the wavelength by which a maximum output is attainable,there occurs only a small drop in output of about a few percent.Generally, in exposure apparatuses, a large decrease in output leads toreduced illuminance on an image plane or degraded resolution, and itcauses a serious problem. However, the small decrease of output found bythe inventors can be easily compensated for by setting the opticalsystem appropriately.

In this embodiment, the design wavelength of the projection opticalsystem 6 and the emission wavelength of the light source means 1 areregistered with the wavelength of a high coherency and a continuousemission laser light source, for example, the wavelength of doubleharmonics (continuous wave) of laser light from an argon ion laser.Namely, the emission wavelength of the excimer laser is adjusted andregistered with the wavelength of light from an argon ion laser. Severalmethods are applicable to registration of the wavelength to these lasers(the argon ion laser is provided with harmonic wave producing means).

In FIG. 3, laser light from an argon ion laser 29 iswavelength-transformed by means of a wavelength transforming device 28,having a secondary harmonic wave producing element, by which a secondaryharmonic wave is produced. This secondary harmonic wave is thenreflected by mirrors 30 a and 30 b, and is it projected to a wave meter3. By using this wave meter 3, the wavelength of the laser light comingfrom the system including the argon ion laser 29 and the wavelengthtransforming device 28 is measured. With the thus detected wavelength,the wavelength of laser light from the excimer laser 1 is adjusted andregistered.

FIG. 4 is an embodiment of an interferometer system, wherein theemission wavelength of a KrF excimer laser for providing pulse lightwith a center wavelength 248.25 nm, to be used in an exposure apparatus,is adjusted and registered with the wavelength of a double harmonics ofa continuous emission type argon ion laser the optical performances ofthe projection optical system 6 is measured. In FIG. 4, the laser lightof a wavelength 496.5 nm from the argon ion laser 29 iswavelength-transformed by a wavelength transforming device 28, whereby asecondary harmonic wave (wavelength 248.25 nm) as the same orsubstantially the same wavelength of the KrF excimer laser is produced.It is then directed to a collimator lens 16 by which it is shaped into adesired beam diameter. Then, the light impinges on a half mirror 25.

The half mirror 25 reflects a portion of the received laser light, andthis portion of the light is collected by a collimator lens 21 and isdirected to the projection optical system 6. The projection opticalsystem 6 collects the received light, and the collected light is thenreflected by a spherical mirror 20 such that the light goes back alongits oncoming path. the light then passes the half mirror 25, and bymeans of an imaging lens 23, the light is projected on the surface of aCCD camera 24, as measurement light.

On the other hand, the remaining portion of the laser light impinging onthe half mirror 25 passes it. The remaining portion of the light,transmitted through the half mirror 25, is reflected by a referencemirror 22 such that the light goes back along its oncoming path. Thelight is then reflected by the half mirror 25 and, by means of theimaging lens 23, it is projected on the surface of the CCD camera 24 asreference light.

The image is formed on the camera 24, based on the two light fluxes ofmeasurement light and reference light provides an interference fringe(resulting from interference between the two light fluxes) which bearsinformation related to aberration, for example, of the projectionoptical system 6. The interference fringe is analyzed by an imageprocessing system 27, by which aberrations and the like of theprojection optical system 6 are detected. Based on this detection, theoptical performance of the projection optical system 6 is measured.Thus, the optical performance of the projection optical system can bewell adjusted on the basis of the stabilized laser light as has beendescribed, such that high resolution pattern printing is assured withthe thus adjusted projection optical system.

Next, an embodiment of a device manufacturing method which uses anexposure apparatus as has been described in the foregoing, will beexplained. In this embodiment the invention is applied to themanufacture of semiconductor devices.

FIG. 5 is a flow chart of the sequence of manufacturing a semiconductordevice, such as a semiconductor chip (e.g., IC or LSI), a liquid crystalpanel or a CCD, for example. Step 1 is a design process for designingthe circuit of a semiconductor device. Step 2 is a process formanufacturing a mask on the basis of the circuit pattern design. Step 3is a process for manufacturing a wafer by using a material such assilicon.

Step 4 is wafer process which is called a pre-process wherein, by usingthe so prepared mask and wafer, circuits are practically formed on thewafer through lithography. Step 5 subsequent to this is an assemblingstep which is called a post-process, wherein the wafer processed by step4 is formed into semiconductor chips. This step includes assembling(dicing and bonding) and packaging (chip sealing). Step 6 is aninspection step wherein an operability check, a durability check, and soon, of the semiconductor devices produced by step 5 are carried out.With these processes, semiconductor devices are finished and they areshipped (step 7).

FIG. 6 is a flow chart showing details of the wafer process. Step 11 isan oxidation process for oxidizing the surface of a wafer. Step 12 is aCVD process for forming an insulating film on the wafer surface. Step 13is an electrode forming process for forming electrodes on the wafer byvapor deposition. Step 14 is an ion implanting process for implantingions to the wafer. Step 15 is a resist process for applying a resist(photosensitive material) to the wafer. Step 16 is an exposure processfor printing, by exposure, the circuit pattern of the mask on the waferthrough the exposure apparatus described above. Step 17 is a developingprocess for developing the exposed wafer. Step 18 is an etching processfor removing portions other than the developed resist image. Step 19 isa resist separation process for separating the resist material remainingon the wafer after being subjected to the etching process. By repeatingthese processes, circuit patterns are superposedly formed on the wafer.

In the embodiments described hereinbefore, the projection optical systemhas a lens assembly. However, it is within the scope of the presentinvention to use a projection optical system having a mirror assembly ora projection optical system having a concave mirror and a lens assembly.

Further, while in the foregoing embodiments, the wavelength of asecondary harmonic wave of an argon ion laser is applied as themeasurement wavelength of an interferometer, if the design wavelength ofa projection optical system differs from that of the embodiments, themeasurement wavelength should be changed with respect to the designwavelength. In that case, the wavelength of a secondary harmonic wave ortertiary harmonic wave of laser light from an argon laser or from anyother gas laser may be used as the measurement wavelength, and thewavelength of a pulse light source to be used for the exposure processmay be adjusted and registered to the measurement wavelength.

Further, in the foregoing embodiments, the invention has been describedwith reference to a step-and-repeat type exposure apparatus (stepper).However, the invention is applicable also to a step-and-scan typescanning exposure apparatus.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. A method of manufacturing a projection exposureapparatus having a pulse laser, said method comprising the steps of:measuring an optical performance of a projection optical system, byproducing an interference fringe which bears information related toaberration of the projection optical system, by use of a harmonic of alaser having a coherency higher than that of the pulse laser; anddetecting the interference fringe, wherein a wavelength of the harmonicof the laser corresponds to a wavelength of light from the pulse laser.2. A method according to claim 1, wherein the pulse laser comprises anexcimer laser.
 3. A method according to claim 2, wherein the excimerlaser includes means for narrowing a bandwidth of the light and forchanging the wavelength of the light.
 4. A method according to claim 1,further comprising adjusting the projection optical system in accordancewith a result of the detection of the interference fringe.
 5. A methodof manufacturing a projection exposure apparatus having a KrF excimerlaser, said method comprising the steps of: measuring an opticalperformance of a projection optical system, by producing an interferencefringe which bears information related to aberration of the projectionoptical system, by use of a harmonic of an Argon ion laser; anddetecting the interference fringe, wherein a wavelength of the harmonicof the Argon ion laser corresponds to a wavelength of light from the KrFexcimer laser.
 6. A method according to claim 5, wherein the excimerlaser includes means for narrowing a bandwidth of the light and forchanging the wavelength of the light.
 7. A method according to claim 5,further comprising adjusting the projection optical system in accordancewith a result of the detection of the interference fringe.
 8. A methodof manufacturing a projection exposure apparatus having a pulse laser,said method comprising the steps of: measuring an optical performance ofa projection optical system, by producing an interference fringe whichbears information related to aberration of the projection opticalsystem, by use of a harmonic of a laser providing a continuous wave; anddetecting the interference fringe; wherein a wavelength of the harmonicof the laser corresponds to a wavelength of light from the pulse laser.9. A method according to claim 8, wherein the pulse laser comprises anexcimer laser.
 10. A method according to claim 9, wherein the excimerlaser includes means for narrowing a bandwidth of the light and forchanging the wavelength of the light.
 11. A method according to claim 8,further comprising adjusting the projection optical system in accordancewith a result of the detection of the interference fringe.
 12. In amethod of manufacturing a projection exposure apparatus having a KrFexcimer laser, the improvement comprising: measuring an opticalperformance of a projection optical system, by producing an interferencefringe which bears information related to aberration of the projectionoptical system, by use of a secondary harmonic of a laser having awavelength of 494.5 nm, wherein a wavelength of the harmonic of thelaser corresponds to a wavelength of light from the KrF excimer laser.13. A method according to claim 12, wherein the excimer laser includesmeans for narrowing a bandwidth of the light and for changing thewavelength of the light.
 14. A method according to claim 12, furthercomprising adjusting the projection optical system in accordance with aresult of the detection of the interference fringe.